Adhesive bonding is a joining technique used in the manufacture and repair of a wide range of products. Along with welding and soldering, adhesive bonding is one of the basic joining processes. In this technique, components are bonded together using adhesives. The broad range of types of adhesives available allows numerous materials to be bonded together in products as diverse as vehicles, mobile phones, personal care products, buildings, computers and medical devices.
History
An adhesive can be defined as a substance that causes two surfaces to stick together. By this definition, the earliest "adhesive" could be considered to have been developed three billion years ago, when primordial cells produced a tacky outer membrane allowing them to stick to adjacent cells. The first use of adhesives by humans can be dated to around 220,000 B.C., when tar from birch tree bark was used to glue stone arrowheads to a shaft.
Basics
According to the definition of EN 923 : "Adhesives. Terms and definitions", adhesives are non-metallic substances capable of joining materials by surface bonding (adhesion), with a bond possessing adequate internal strength (cohesion)". The adhesive forms the connecting element between the two joined parts, which would not stick together without it. Adhesives can be grouped by chemistry, by application, or by the reaction mechanism.
Adhesion According to IUPAC, adhesion is the "process of attachment of a substance to the surface of another substance". Interactions between the adhesive and substrate have a very short range of less than one nanometre. Therefore, good wetting of the materials to be joined by the adhesive in its liquid state is required to produce a high quality bond. In addition to the wetting ability, the adhesive and substrate must have compatible molecular groups so that interaction between the adhesive and substrate can take place and thus achieve adhesion.
The adhesive forces are usually based on physical interactions, for example, such as those between polar or polarisable groups, on hydrogen bonds, or van der Waals forces. When bonding plastics, in particular with solvent-based adhesives, diffusion processes can also play a role. In this case, the plastic at the substrate surface is dissolved by the solvent contained in the adhesive. This leads to an increased mobility of the plastic's polymer chains, which in turn allows penetration by those of the adhesive. Ultimately, additional interactions occur between the polymer chains of the adhesive and the substrate. After evaporation of the solvent, a solid compound is formed. Chemical bonds are also important in certain adhesive / substrate combinations, for example when bonding glass using silicone adhesives, wood using polyurethane adhesives and aluminium using epoxy adhesives. Chemical bonding leads to significantly higher adhesion than physical bonding. In addition, penetration of the liquid adhesive into undercuts may provide addition adhesion after it has hardened.
Achieving adhesion between the adhesive and substrate requires not only an adhesive of suitable composition for the substrate, but also places high demands on the substrate surface. Due to the short range of the adhesion forces, the nature of the surface layer of the substrate is crucial. It must be sufficiently firmly connected to the body of the substrate. For example, many adhesives adhere well to a corroded steel surface. However, the corrosion layer – the rust – is not firmly connected to the substrate. Under load, failure may occur in the corroded material or between the rust layer and the uncorroded steel. The same applies to coated items. The adhesive must build adhesion to the coating. The coating in turn must be sufficiently firmly connected to the substrate.
Likewise, contaminants, especially those which, due to their low surface tension, counteract wetting by the adhesive (for example, oils, release agents, etc.) hinder the adhesion interaction. Contaminants form, as it were, a barrier between the adhesive and the substrate which cannot be bridged by the adhesion forces due to their short reach.
Therefore, contaminants usually need to be removed before adhesion. Some special adhesives show a degree of compatibility with certain oils. They are able to absorb certain oils during the curing of the adhesive, which takes place at elevated temperatures, and thus to remove them from the boundary layer between the adhesive and substrate. Such adhesives are used for example in automotive body shops. They allow the gluing of sheet metal parts with corrosion protection and drawing oils without previous cleaning; The curing of the adhesive takes place in the furnaces used subsequently for hardening the lacquer at temperatures between approximately 150 and 200 °C.
Pre-treatment Pre-treatment can be used to modify surfaces in a targeted way and thus make them more adhesive. In addition to coating the substrates with an adhesion promoter (primer) to enable good adhesion, surfaces can also be modified by various methods to prepare them for gluing. The most common surface pre-treatment methods are listed in the adjacent figure.
The selection of the pre-treatment process is application-specific, taking into account The materials to be joined. Their surface condition. The type and amount of surface contamination. The adhesive to be used for bonding the substrates. The stresses on the glued product over its life cycle (e.g. mechanical, thermal or medial).
The selection should be validated through appropriate testing.
Hardening of the glue – cohesion As the adhesive solidifies, its internal strength, the cohesion, increases. The cohesion is also based on physical interactions, in this case between the adhesive polymers. In the case of adhesives which cure by a chemical reaction, i.e. the formation of polymers by a chemical reaction of the adhesive constituents, the resulting chemical bonds play an important role.
Properties of a bond The cohesive and adhesive properties of the adhesive in combination with the substrate determine the properties of a bond. While the adhesion properties substantially determine whether an adhesive adheres to a particular substrate, the cohesive property contributes greatly to the mechanical properties of the bond, particularly the load-bearing deformation behaviour.
Adhesive bonds are not only subject to certain aging characteristics, but their properties are dependent on the particular environmental conditions, in particular the temperature. Also, both the adhesion-forming interactions between adhesive and substrate, as well as the inter-intramolecular interactions causing the cohesion, can be adversely affected by external influences (including temperature, humidity, chemicals, radiation, mechanical stress). The degree of impairment depends on the nature of the conditions and their duration; this process is called aging. Therefore, when planning a bonding operation, in addition to the actual environmental conditions, their possible long-term effects on the adhesive and substrate must also be taken into account.
Due to the large number of parameters that can influence bonding and the partly conflicting requirements for different adhesive bonds, it is clear that the so-called "all-purpose adhesive" cannot exist.
Adhesive selection
The selection of a suitable adhesive for a particular application should be based on a specific requirement profile. This requirement profile lists all immediate, verifiable requirements for the component to be bonded and, as a result, for the bond and the adhesive. It is possible to distinguish between requirements which must be met and those whose fulfilment is advantageous but not absolutely necessary. In addition, specifications derived from the bonding process, including those from upstream and downstream process steps, must be taken into account. The diagram above provides a summary of the most important parameters to take into account when choosing adhesives.
Advantages and disadvantages of gluing As with any joining technique, bonding technology not only offers a wide range of possibilities and many advantages, but also has limitations that must be considered when planning and designing adhesive processes.
Advantages of gluing The main advantages are: Almost all materials can be bonded to themselves or other materials by gluing. Due to the areal force transmission, a uniform distribution of the forces is achieved over the entire bonding area. This makes optimum use of the substrate properties. By choosing a correspondingly large bonding area, relatively high forces can also be transmitted between thin substrates, which is particularly advantageous for lightweight construction applications. By choosing an elastic adhesive, movements of the substrates relative to each other can be compensated for. For example, it is possible to compensate for component elongations in the event of temperature fluctuations and to dampen vibrations effectively, thus avoiding material damage or fatigue of the substrate. There is no material damage to the substrate through drill holes for rivets or screws, etc. Visually appealing surfaces can be achieved. Zero or only a slight heat load is associated with the bonding, so that thermal distortion, thermal stresses or changes in the microstructure and, as a consequence, a change in the mechanical properties of the substrate are largely avoided. Substrate tolerances can be compensated for by means of gap-filling adhesives Gluing is equally suitable for both small and large parts. Thus, in microelectronics only a few micrograms of adhesive are required per component, while in the production of rotor blades for wind turbines, several hundred kilograms per component are needed. In addition to power transmission, additional properties can be incorporated, such as: Connection sealing. Acoustic decoupling and damping. Electrical insulation (avoidance of contact corrosion) Electrical conductivity. Thermally insulation. Thermally conductivity (for example, thermal management of electronic components).
Disadvantages There are many types of adhesive available and they are often developed with a specific use in mind. As a result, what could be perceived as a disadvantage of a particular adhesive type in some applications could be its advantage in another. Therefore, it is essential to use a glue that is appropriate for the application at hand. Characteristics of adhesives that can be disadvantages in certain situations include: In general, instantaneous bond strength is not achieved. However, pressure-sensitive adhesives used in double-sided adhesive tapes, rapidly curing "superglue" cyanoacrylate, and many light-curing adhesives do achieve significant initial strength after completion of the joining process or very shortly thereafter, although not their ultimate strength required for further processing of the glued assembly. Depending on the chemical basis, some adhesive types have limited thermal and chemical resistance and the mechanical properties of the bond are temperature dependent. Therefore, choose a type of adhesive that has been developed to have good thermal and chemical resistance, such as reactive hot melt adhesives. Some adhesives may exhibit a slight tendency to creep. Choose an adhesive designed not to creep if this is important for your application. The long-term stability of a bond is subject to aging processes, so make sure you choose an adhesive with a lifespan appropriate to your application. Some types of adhesive cannot be removed without damaging at least one of the substrates. Others are easier to remove.